Joshua Rosenau spends his days defending the teaching of evolution at 
Reposted from the old TfK in honor of this story at BoingBoing.
Botflies are disgusting. Horrible, horrible little things. We get their larvae on the white-footed mice we trap here in Kansas. The picture to the right is a white-footed mouse botfly that I caught here in Kansas.
These are fairly rare in collections because the adult is only active for a short time. Many species lack mouthparts because of their short adult life. It flies around, it mates, and the females lay eggs on grass stems. The mouse brushes against the egg, the heat of the body causes the egg to hatch, the larva burrows into a natural body opening (open wound, mouth, nose, anus). For 5 days, it crawls under the skin until it reaches a suitable site and builds a little home where it grows. That little home is called a bot or a warble (bot flies are also called warble flies).
Parasites are like hackers, they look for ways to make a system work against itself. The botfly larva causes the body to secrete various wound-fighting serums, which it happily consumes, growing and causing the body to produce more and more serum. A larva can grow 22 mm long in a mouse that's only 65 mm long (ignoring the tail).
In masters study conducted here at KU, a student found that between 20 and 30% of white-footed mice in the study area were infected, though at peak periods, almost 9 out of 10 were infested. A quarter of infested mice had more than one bot. There was no effect of sex, but older mice were more likely to be parasitized than younger mice.
In the next set of images, you can see a warble on a human leg. Burt Humburg, ally of science extraordinaire, kindly provided the video from which I took the screengrabs (warning: slight profanity, disgusting larva, and the site where the video is hosted takes ads from porn sites. I haven't tested the video recently). The first image shows the warble, a big bump on the leg with a hole. If the video were higher resolution, you'd see the tail of the larva wiggling around. The second image shows the tweezers someone is using to try to drag the larva out, which you can see in the lower left. The remaining images show the horrific beast out of the flesh. (Note: Doctors advise against attempting to yank the larva out as shown above, there are spines that the larva holds itself in with, and pulling is likely to leave parts of the beast in the warble. This is disgusting, and can also cause infection.)
I don't know the origin of the video, so I can't speak to the species of botfly in question. Odds are good that it's a tropical species, since the number of human infestations by North American botflies is vanishingly small. Central American species parasitize humans with greater frequency. One man chronicled his adventures with a bot that grew on his scrotum.
People with farms may be familiar with a very different subfamily of botfly than the one shown above. The genus Gastrophilus (stomach lover) will dive-bomb horses and mules, laying eggs on their fur. The eggs hatch, and as the horse grooms itself, it winds up ingesting the larvae which grow in the gut (hence the name). Horses, not surprisingly, don't enjoy the slow progress of the larva through their digestive tracts, and respond violently when they hear a botfly coming.
Science isn't always pretty, and pretty much anyone who knows anything about botflies thinks that they are the most disgusting things they've ever seen. Indeed, I'm told that the author of a popular book about parasites passed on blogging the video. There's something mildly cute about an isopod taking over as a tongue for a fish, but there's nothing cute about tiny wiggling things poking out of your skin.
But this is what makes biology fun. There are exciting, fascinating and unexpected discoveries awaiting us under barks, rock, and even skin.
Comments
Every tropical biologist I have heard speak on the matter remark that getting their first botfly is some kind of right of passage. So much so, that one made a point of not mentioning their infestation to Customs so that they could safely get their trophy home.
Posted by: Jason | July 19, 2007 8:43 PM
I have a friend doing a PhD in entomology, and he says he knows several people who have "raised their own fly" as a kind of initiation into a lab...
Posted by: Anne-Marie | July 19, 2007 11:03 PM
I live in Kansas, is it possible to get a bot fly larva parasite here?
Posted by: jpe | July 20, 2007 2:19 PM
Horses can, rabbits and mice can, but I'm not aware of humans ever getting bots in Kansas. Someone told me they got infected in Washington state, but I think that's a rare situation. The bot that most frequently infects humans is found in Latin American rainforests.
Posted by: Josh Rosenau | July 20, 2007 3:40 PM
"Is there a parasitologist, past or present, in the house?"
Why yes there is--glad to comment.
I haven't watched the video clip, and don't plan to, mostly because these parasites really aren't that big a deal. I guess my squeamish index is pretty high. There was a tidbit in the news a few days ago about a guy in Colorado (is this the post at BoingBoing? I haven't looked at it yet) who felt something moving on his scalp and he had several Dermatobia hominis larvae under the skin, doing their usual thing. In the tropics, this is common stuff, certainly not worthy of reporting in the news.
Now, a few clarifications.
1. Several bot fly (BTW, it's two words) species exist in North America, and they're pretty host-specific, although there are occasional reports of human infestations with each (probably more so with Cuterebra spp.; item 3 below). Sheep: Oestrus ovis; bots develop in the sinuses; horses et al: Gasterophilus spp.; bots develop in the stomach (and actually hang on to the lining); cattle: Hypoderma spp.; bots develop under the skin; rodents, rabbits: Cuterebra spp; bots develop under the skin; deer, elk, other wild cervids: Cephenomyia spp.; bots develop in the sinuses, similar to Oestrus ovis in sheep. In all these host species, the presence of the bots causes very few problems, if any.
2. Cats, and less often dogs, are often infected with Cuterebra spp. Every practicing veterinarian has seen this in mid- to late summer. Usually, the owner notices a small hole on the side of the cat's neck (and sometimes worries that it's been shot with a BB gun). Veterinarian smiles knowingly and informs the owner that it's an easy problem to fix. Depending on the cat (and the owner), it's often possible to fish the thing out right in the exam room, and with a few days of warm compresses and a week or so of oral antibiotics, they almost always heal up fine. The larva is usually quite small (less than 1 cm) at this point, although I have seen one cat that had a full-sized (as in ready to pop out any second)larva under its skin, and it was about an inch long, black, spiny and as thick as my index finger. I didn't treat that animal myself, but it was removed (it would have come out on its own anyway, just as they do in rodents and rabbits) and the cat was fine.
3. As I said, people are rarely infested with any of these animal bot fly larvae, but Cuterebra seems to happen most often. And it can happen in Kansas or Washington as easily as anywhere else. I have several published case reports in my files if anyone wants the specifics. Dermatobia hominis is the most common cause in people (and many animal species) in Central and South America, but it's not native to the U.S.
4. The condition of being infested with bot fly larvae is termed myiasis. Brevity is sweet.
5. The appearance of the adult Cuterebra fly (photo above) is similar to all the bot fly species. They look terrifying, but if you look closely, they have vestigial mouthparts--i.e., no biting. They're loud and annoying, but nothing else.
6. Finally, one very minor point: horses have hair, not fur, although some of them can look pretty furry after being out on the range all winter :).
Forgive me if I got up on my lecturer soapbox here, but this post was too good to pass on :).
Posted by: Bob, DVM | July 22, 2007 11:25 PM
Bob, thanks for adding your insights. Do you happen to know which species of botfly infects/infests/affects/myiasizes cats in the US? Is it possible they get infected from mice/rabbits they swallowed? I'm sure that'd be hard to document, but interesting.
I can't say I think the adult fly looks terrifying, I think it's handsome. The maggots are hideously disgusting, and I know professional biologists who squirm at the mere words "botfly."
I am especially grateful for reminding me of the proper term. "Myiasis" applies to maggots feeding on flesh in general, not just to botflies, of course.
Posted by: Josh Rosenau | July 23, 2007 1:22 PM
Josh-
Glad I could chime in. Virtually all feline infestations involve Cuterebra spp. (they're not identified past genus level). I'm sure that a few, rare published reports of other bot fly infestations in cats are out there, but I've never seen them or heard of them, and my myiasis file is pretty complete.
Cats/kittens are thought to be infested mainly by brushing up against vegetation where the Cuterebra eggs have been laid, and the microscopic larvae (hatched out after being bumped) crawl into ears/mouth/nose, penetrate the tissues and migrate eventually to the sides of the neck. It's thought that ingesting the eggs (attached to a rodent/rabbit)could also do the trick). Not sure if ingesting a mouse which already has a developing larva in its body would be a route, but it probably would if the larva were small.
Almost always, the larva (and it's a single one, in 99.999% of cases) ends up on one side of the neck and creates a small breathing hole, which is what the owner notices. This location supports the idea that nearly all infestations are acquired through the orifices on the head. If skin penetration by the larva (or the fly laid eggs on the cat, which then hatched and penetrated the skin), logically, you would find the bots anywhere on the body, which almost never happens. Rarely, the larvae make a wrong turn during their migration through the pharyngeal tissues and instead of reaching the skin, they migrate into the pharynx and/or trachea, sometimes with serious results.
The question of skin penetration is debated still, although most people agree that the above description is how most cases happen. Interestingly, there's a report from your state (J Kans Ent Soc 31:67-71, 1958) in which the author allowed Cuterebra larvae to briefly penetrate his own skin, just to see if it would happen. (It did, but he removed the larvae almost immediately--"There was no intent to determine possible further development in man.").
Ah, science in the 50s...........
Posted by: Bob, DVM | July 23, 2007 10:56 PM
Hi! Just browsing your site after removing a very large warble/larvae from my dog. One of the most disgusting things I've seen. It was quite large and I basically squeezed it out with no real problems, it did leave behind quite a big hole. The dog seems fine , thank you for such a resourceful site. Cheers, from Canada!
Posted by: sharon flaman | August 3, 2007 8:49 PM
Thank you Bob DVM! This has been the most informative site I've found for information on Bot flies in cats. I have a feral mother with 4 kittens that I have been trying to tame and then catch. All 4 of the kittens have bot flies! At first I thought they had been attacked by another cat. Then my neighber told me she thought they were bot flies. I think one kitten was attacked by soemthing as the back of it's left ear down to it's neck looked pretty ripped up. This kitten now has the biggest warble in the damaged area. My question is how long does it take for the larva to develop and finally fall out? Is it weeks or months? I know I'm going to have to take these kittens to the vet as soon as I can catch them I just don't want to suddenly find all these larva in the cage :( Thanks!
Posted by: PeggyH | August 7, 2007 11:48 AM
Peggy--
Sorry for the delayed response--it seemed like this thread was closed out, so I haven't looked at it in a while. Normally the bots pop out on their own after a few weeks (certainly not months), and they will be full-sized by that point, so brace yourself for a shock. But remember--the holes almost always heal up without complications. And if you do find the larva(e) on the floor of their cage, just toss them in rubbing alcohol if you want to zap them (or save them for a school science fair project :)
Of course, if you've managed to take these little guys to your veterinarian, all this is moot.
Posted by: Bob, DVM | August 16, 2007 11:11 AM
Hi,
I'm a student at a well known University in Washington DC. Recently, my roommate and I moved in to an apartment in an attempt to save money that was filled with bed bugs. After having these exterminated everything seemed to be okay until a few weeks ago when I realized I had one of these tiny maggots living under my skin. I surgically removed it myself and brought it to my student health services. The doctors and nurses were completely surprised, and outside of a Doctor from South America none of them had even heard of botflies. They removed it but really couldn't provide much insite. There are a ton of cats and dogs that live in my building and I share many of the same facilities as their owners do. Was this a human bot fly or a cat bot fly? The discharge was a mass of pearl white particles (foul odored). The wound was on my side around my belt line. The specimen, after removed was about 1 cm in length and about .3 cm in diameter. Due to the Doctor's inexperience in the field, and since I did most of the removal I'm worried that I only removed it partially. It's been almost two weeks and I still have a red lump (without the hole and its not pussing) on my side. It's dense. The specimen was covered in blood but it did seem to be segmented. So, in summary
-- 1 cm long, .3 cm in length. -- white mash potatoe-like discharge with door --cats and rodents in building, and bed bugs prior to infection --noticed lump while it discharged and was painless, removed it two weeks later while it was painful
1) Was this a cat/rodent bot fly, a human bot fly or something else?
2) How rare are these cases? Will this happen again?
3)Do you think it was due to my apartment?
Thanks
Ryan Kelley
Posted by: Ryan | December 13, 2007 4:16 AM
Your description doesn't sound altogether botlike to me, but that's a determination that a real doctor should make. Removing a bot yourself is never wise, since the chance of infection is high. Botflies are fairly noticeable, so you would know if they were flying around inside your pad. Human bot flies are tropical, so I doubt that's what you're dealing with.
If the folks at the student health services can't help, get a referral to a dermatologist. It could be that you had some sort of pimple, boil or infection right at the beltline. A bot isn't impossible, but it isn't likely, either. Trust your docs.
Posted by: Josh Rosenau | December 13, 2007 6:59 PM